OBJECTIVES: a) Overall objectives of the total project. As stated in the original application,"...the long term goal of this project is to provide information about the mechanisms involved in neural differentiation, the genesis of neuronal patterns and the formation of specific nerve connections." b) Objectives for the current year. During the current year we have devoted our efforts to explore the formation of photoreceptor patterns in the compound eye of Phaenicia (Diptera, Calliphoridae). It was planned and stated in the original application that one of the short term goals of the project would be answering the following question: "At what developmental stage does the asymmetrical cell pattern characterizing the ommatidia of Dipterans appears.?" Our studies, together with those by Ready et al. have demonstrated that the proto-ommatidia of third instar larvae already show an asymmetrical distribution of their cells.